As is well known, the increasingly higher operating rates required of modern looms mean that the grippers for inserting the weft into the shed are required to undergo continually increasing simplification combined with increasing sophistication of their gripping devices which, ideally, should not only maintain a controlled loading along the entire gripping region but should also allow easy cleaning between the gripping members, which should therefore be able to be easily withdrawn from each other. Various types of weft yarn gripping devices for the grippers of shuttleless looms are already known in the current state of the art.
In one of these types a gripping foot, which can be rigid or flexible, is positioned above, but parallel to and in line with, a rigid gripping surface against which said foot is pressed by an overlying leaf spring.
Such a device, by providing perfect alignment between the foot and the relative gripping surface, enables the required load distribution to be obtained along the entire gripping region, but has the drawback of not allowing the foot to be cleaned as it cannot be raised.
To overcome this drawback another type of gripping device has been used in which said gripping foot is rigid with the end of a lever on which a spring acts to press the foot onto the underlying rigid gripping surface.
In this manner, on rotating the lever against the action of its spring, the foot is withdrawn from the gripping surface, making it possible to clean the inner gripping region. However, this new method produces misalignment between the mobile foot, which rotates about the lever pivot pin, and the gripping surface due to the inevitable constructional tolerances of the various components which, even if minimal, are always discernible.
To enable the mobile foot to perfectly adapt to the relative gripping surface to obtain the required load distribution along the entire gripping region, a thickness of elastic or elastomer material is now usually interposed between the foot and gripping surface and fixed to this latter, its purpose being to absorb said misalignment, but which is possible only if the foot has a certain rigidity and if the lever spring acts with a large force, this also enabling the negative effects of vibration to be also absorbed.
From the aforegoing it is apparent that such a gripping device not only has the drawback of a limited life in that the high spring load acting on the elastomer in the long term deforms the entire elastomeric gripping surface, but is also absolutely unsuitable for extremely weak yarns for which said load would be excessive, and in addition the much greater tapering of the foot required for weak yarns makes the foot very flexible, so that there is no longer a reliable control of the load pressing against the underlying elastomeric surface as said load varies along the gripping region, and the high flexibility of the foot means that it cannot be guaranteed to press uniformly against the elastomer along its entire contact length. The contact load is therefore uncertain as it is not adequately distributed over the entire gripping region.